Method and apparatus for chromatic dispersion measurement based on optoelectronic oscillations

ABSTRACT

In this invention, a novel technique is introduced to measure chromatic dispersion (CD) in optical fibers. This technique is based on a relatively low-frequency optoelectronic oscillation (OEO) to provide fast, precise and low cost method for CD measurement that can be implemented easily in commercial instruments. In addition, another technique is presented to compensate for fiber thermal fluctuations during measurement which is based on a second simultaneously oscillating OEO. The proposed setup is implemented to measure the CD in normal single mode fibers with lengths of 40 km, 10 km, 1 km. Moreover, it is implemented to measure CD in 400 m of nonzero dispersion shifted fiber to test the system ability to resolve small chromatic delays. The proposed setup can resolve delays less than 0.1 ps/nm (which can be further improved by increasing the oscillation frequency) and measure CD with precision as low as 0.005 ps/nm.km as low as 20 seconds over a wavelength range from 1500 to 1630 nm. Further improvements may be possible by slightly better system design.

REFERENCE CITED

U.S. Pat No. 4,752,125 A June 1988 Schicketanz JP 2000081374 A March2000 Hiromasa Ito et al. U.S. Pat No. 6,912,04 B June 2005 JirgenBrendel US 2013/015642 A June 2013 Emilio Bravi U.S. Pat No. 7,787,12 BAugust 2010 Michael Galle U.S. Pat No. 5,406,368 A April 1995 Horiuchiet al. U.S. Pat No. 5,033,846 A July 1991 Hernday et al.

OTHER PUBLICATIONS

K. S. Abedin, M. Hyodo, and N. Onodera, “Measurement of the chromaticdispersion of an optical fiber using a Sagnac interferometer employingasymmetric modulation”, Opt. Lett., 25, pp. 299-301 (2000).

K. S. Abedin, “Rapid, cost-effective measurement of chromatic dispersionof optical fibre over 1440-1625 nm using Sagnac interferometer”,Electronics Letters, vol. 41, No. 8 (2005).

FIELD OF INVENTION

The present invention is related to a novel measurement technique forchromatic dispersion of single mode fibers based on optoelectronicoscillations.

BACKGROUND

Nowadays, transmission bandwidth has been increased in long-haul opticaltransmission systems from 2.5 Gbit/second to 10 Gbit/second and soon to40 Gbit/second. Higher bandwidth means that the transmitted opticalpulses become near to each other and can overlap if they experiencesufficient chromatic dispersion. Therefore, chromatic dispersionmeasurement of long-haul network is of a great importance to ensureproper operation of such networks.

Tremendous efforts have been spent to find suitable method for chromaticdispersion measurement. Among those methods the Time-of-Flight, theModulation Phase Shift, and the Interferometric method are recommendedby the International Telecommunication Union (ITU-T G.650) and by theInternational Electro-technical Commission (IEC 60793-1-42:2013).

Although the time-of-flight technique (U.S. Pat. No. 4,752,125 bySchicketanz) is simple to implement, it has low accuracy and is notsuitable to resolve small chromatic dispersions.

The modulation phase shift technique became an industry standard andcovered by several patents (references: U.S. Pat. No. 5,033,846 byHernday et al., U.S. Pat. No. 5,406,368 by Horiuci et al.). Thistechnique has better accuracy than the time-of-flight technique,however, it is time consuming and expensive to implement since it needsan expensive network analyzer. An example of a commercial device thatimplements this technique is the Agilent 86038C.

The best chromatic dispersion measurement accuracy can be obtained fromthe Interferometric technique (U.S. Pat. No. 7,787,12 by Michael Galle);however, it can only measure short fibers of lengths in the order of onemeter.

Further techniques have been investigated to provide fast operation andhigher accuracies with less complex system. A ring-type Sagnacinterferometer has been proposed to measure chromatic dispersioncost-effectively (K. S. Abedin et al. Opt. Lett., (2000)); however, thistechnique is time-consuming due to the time required for the analysis ofthe acquired fringes at every wavelength. Although further improvementhas been made to this technique to make the measurement timeconsiderably smaller (K. S. Abedin, Electronics Letters, (2005)), thechromatic dispersion measurement through voltage change degrades itsaccuracy and makes the traceability to the SI unit of time not easilypossible.

Therefore, a need still exists for a technique that is: precise, fast,low-cost and traceable to the SI unit of time for chromatic dispersionmeasurement.

BRIEF SUMMERY OF THE INVENTION

The Present invention comprises a novel technique for chromaticdispersion measurement. This technique is based on creating a relativelylow-frequency optoelectronic oscillation (OEO), in which theelectro-to-optic converter is a tunable laser source. In order tomeasure chromatic dispersion, the tunable laser is swept over thewavelengths range of interest, while change in the oscillation frequencyof the optoelectronic oscillator is measured. Consequently, thechromatic dispersion can be calculated from the change in oscillationfrequency and the change in wavelength.

An additional optoelectronic oscillator (OEO) can be used with the mainoscillator simultaneously to compensate for the thermal fluctuations inthe fiber under test, which can greatly affect the results if the fiberunder test is not in a stable weather conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

embodiments of the present invention are illustrated as an example andare not limited by the figures of the accompanying drawings, in whichlike references may indicate similar elements and in which:

FIG. 1—FIG. 1 depicts the basic scheme for chromatic dispersionmeasurement using optoelectronic oscillation (OEO) (TL: tunable laser,MZI: Intensity modulator, FT: Fiber under test, PD: Photodiode, AMP: RFamplifier, BPF: Band-pass filter, FC: Frequency counter).

FIG. 2—FIG. 2 depicts the complete chromatic dispersion measurementsetup with thermal compensation scheme using optoelectronic oscillation(OEO). DFBL: laser at 1310 nm, TL: Tunable laser, WM: Wavemeter, MZI:Mach-Zehnder Intensity modulator, PD: photodetector, FC: Frequencycounter, SA: RF spectrum analyzer, BPF: Band-pass filter, AMP: RFamplifier, PC: Computer.

FIG. 3—FIG. 3 depicts the sensitivity measurement of the optoelectronicoscillation (OEO) fundamental frequency at 1310 and 1550 nm to thethermal fluctuations on the fiber under test of 10 km length. (Dashedline: 1310 nm, continuous line 1550 nm)

FIG. 4—FIG. 4 depicts the spectrum of Optoelectronic oscillation near 56MHz with sidebands (Fiber under test length=10 km).

FIG. 5—FIG. 5 depicts the chromatic dispersion measurement results at 5nm scanning steps over wavelength range 1500-1630 nm for normal singlemode fiber of length of 40 km STD: standard deviation, Oscillationfrequency=56 MHz.

FIG. 6—FIG. 6 depicts the chromatic dispersion measurement results at 5nm scanning steps over wavelength range 1500-1630 nm for nonzerodispersion shifted fiber of length of 400 m STD: standard deviation,Oscillation frequency=56 MHz.

FIG. 7A—FIG. 7A depicts the chromatic dispersion measurement of 1 kmfiber with 1 nm scanning steps over wavelength scanning range from1500-1630 nm at oscillation frequency of 56 MHz. (STD: standarddeviation).

FIG. 7B—FIG. 7B depicts the chromatic dispersion measurement of 1 kmfiber with 1 nm scanning steps over wavelength scanning range from1500-1630 nm at oscillation frequency of 900 MHz. (STD: standarddeviation).

DETAILED DESCRIPTION OF THE INVENTION

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention.

Novel chromatic dispersion measurement technique is discussed herein.This technique is based on the optoelectronic oscillation (OEO). Inaddition, a technique for compensation of the thermal fluctuations inthe fiber under test by using another optoelectronic oscillator ispresented.

The basic oscillator comprises a tunable laser (TL) (1), an intensitymodulator (MZI) (1), fiber under test (FT) (3), a photodetector (PD)(4), an amplifier (AMP) (5), a filter (BPF) (6), power splitter (7) anda frequency counter (FC)(8) which are connected as shown in FIG. 1).

The RF amplifier (5) should provide sufficient gain to compensate theloss inside the loop and therefore starting the oscillation. The basiccondition for the OEO oscillation is that the accumulated phase aroundthe loop in the optical and RF part to be integer multiples of 2π.

The oscillation frequency of the OEO cavity can be described by thefollowing equation:

$\begin{matrix}{{f_{q} = {{qf} = {\frac{q}{\tau} = \frac{q}{\tau_{F} + \tau_{sys}}}}},\mspace{14mu} {\tau_{f} = \frac{nL}{c_{o}}}} & (1)\end{matrix}$

Where, τ_(F) is the time-of-flight of the light inside the fiber undertest, τ, τ_(sys) are the delays inside the whole cavity and inside themeasurement system respectively, L is the length of the fiber undertest, q is the oscillation mode number, f is the cavity fundamentaloscillation frequency, c_(o): the speed of light in vacuum (299792458m/s), n: the refractive index of the fiber under test which is 1.4682 at1550 nm.

The chromatic dispersion coefficient (D) is defined as the change in thetime-of-flight of the light inside the fiber under test (dτ_(F)) as itswavelength changes by (dλ):

$\begin{matrix}{{D\left( {{ps}\text{/}{{nm} \cdot {km}}} \right)} = {\frac{d\; \tau_{F}}{d\; \lambda \; L} = \frac{q\; {{df}_{q}({Hz})}\mspace{11mu} \left( 10^{12} \right)}{d\; \lambda \mspace{14mu} ({nm})\mspace{11mu} {L({km})}\; f_{q}^{2}\mspace{11mu} \left( {Hz}^{2} \right)}}} & (2)\end{matrix}$

Therefore, by changing the wavelength of the tunable laser by (dλ) whilemeasuring the change in the OEO oscillation frequency (df_(q)), D can becalculated from equation (2).

Therefore, by changing the wavelength of the tunable laser whilemeasuring the change in the oscillation frequency of the OEO, D can becalculated from equation (2).

The setup shown in FIG. 2 is constructed to measure chromatic dispersionand to compensate for temperature drifts using two simultaneously OEO.The chromatic dispersion measuring oscillator comprises of a tunablelaser (TL) (9), a Mach-Zehnder Intensity modulator (MZI) (21),Photodetector (PD1) (18), an amplifier (AMP1) (19), bandpass filter(BPF1) (20), frequency counter (FC1) (15), fiber under test (25) and awavemeter (WM) (10). A RF spectrum analyzer (SA) (16) is used tocharacterize the beat resulting from the oscillation. A computer (22) isused to control the sweeping of the tunable laser and take reading fromthe wavemeters and the frequency counters at each wavelength.

The second OEO, which is used to compensate the thermal drift of thefiber under test (25) during measurement, consists of a laser at 1310 nm(DFBL) (11) (or any different wavelength), another similar photodetector(PD2) (17), amplifier (AMP2) (13), bandpass filter (BPF2) (12) andfrequency counter (FC2) (14).

The light from the tunable laser (9) is directed to the MZI (21). Thelight after the MZI (21) is sent through the fiber under test togetherwith the light from the DFB laser (11) using a fiber combiner (24). Thetwo beams are separated again using a 1310/1550 WDM multiplexer (23), sothat the light from tunable laser falls on PD1 (18) while the light fromthe DFBL (11) falls on PD2 (17). Two RF filters (12, 20) are used toselect the oscillating frequency. The RF amplifiers (AMP1, AMP2) (19,13) are used to compensate the losses in the optical and electricalrouts to maintain the oscillation. The frequencies are counted using thetwo frequency counters.

The RF spectrum analyzer (16) is used to characterize the oscillationbeat and to measure the fundamental frequency by measuring themode-spacing as shown in FIG. (3).

The exact wavelength of the tunable laser is measured continuously usingan accurate wavemeter (10).

Since the wavelengths 1310 nm, 1550 nm have different sensitivity totemperature, a test can be made to find this ratio. A 10 km fiber isplaced into temperature controlled champer and a temperature change ofaround 15° C. is made while measuring the OEO oscillation frequencies ofboth lasers. The measurement results are shown in FIG. 3. The ratiobetween the two fundamental frequencies is found to be

$\frac{f(1550)}{f(1310)} = {0.9896.}$

Therefore, it is possible to compensate the thermal effects on theoscillation frequency of the tunable laser by using the oscillationfrequency of the 1310 nm laser multiplied by this ratio.

According to equation 2, the oscillation mode number has to bedetermined for each fiber under test (25). This number can be determinedeasily from the RF spectrum of the optoelectronic oscillation bydividing the oscillation frequency by the spacing between twoconsecutive peaks which represents the fundamental frequency, see FIG.4.

The setup in FIG. 2 is verified for measuring the chromatic dispersionof 40 km of normal single mode fiber and 400 m of nonzero dispersionshifted fiber. The measurement is performed by sweeping the tunablelaser over the wavelengths from 1500 nm to 1630 nm in steps of 5 nm,while measuring the oscillation frequency change using a frequencycounter. The laser wavelength during sweeping is measured using accuratewavemeter. The chromatic dispersion is calculated from equation (2) andthe measurement results are shown in FIG. 5 (40 km) and 400 m (FIG. 6).

For long fibers, the mode number q is large enough to resolve CD withprecision as low as 0.005 ps/nm.km in step of 5 nm (0.018 ps/nm.km instep of 1 nm) with such relatively low oscillation frequency (56 MHz).However, for short fibers, higher oscillation frequencies are requiredto reach comparable mode number and consequently reach similarprecision. For example, for 40 km fiber, q≈11000 at 56 MHz; on the otherhand, for 1 km fiber, q≈285 at 56 MHz, while it is q≈4583 at 900 MHz.Therefore, in order to enhance the measurement precision for shortfibers, higher frequencies is required. FIG. 7 shows a test measurementmade on 1 km fiber at two oscillation frequencies, namely, 56 MHz and900 MHz. The test shows that at oscillation frequency of 900 MHz themeasurement precision is better than that of measurement at 56 MHz.

When comparing the proposed setup with the best available commercialmeasuring device currently available (ex. Agilent 86037C),optoelectronic oscillation setup is 3 times faster than Agilent since itmeasures chromatic dispersion from 1500 to 1630 nm in 5 nm steps in 20seconds, while Agilent measures it in around 1 minute. The measurementresolution for Agilent system reported to be 0.1 ps/nm which is similarto the proposed setup herein which is 0.09 ps/nm (obtained from the 400m nonzero dispersion shifted fiber measurement) for the low modulationfrequency of 56 MHz. However, by increasing the oscillation frequencythe resolution is expected to be much better depending on the frequencyselected. The price of the OEO system should be much lower than theAgilent system, since the Agilent system employ a vector analyzer tomeasure the phase change which is more expensive than the component ofthe proposed setup herein. The proposed setup can be reduced to simplescheme (like the setup in FIG. 1) if the fiber is placed in atemperature controlled chamber or the measurement is made in relativelystable environment (which is not considered in any setup else). Thespectrum analyzer is needed only once at manufacturing process tomeasure system frequency.

1- Chromatic dispersion measurement method and apparatus usingoptoelectronic oscillations with the fiber under test in its cavity. 2-Thermal fluctuation compensation using additional optoelectronicoscillator that has the same fiber under test in its cavity.